1. Field of the Invention
The present invention relates to an apparatus for ejecting droplets such as an ink-jet printer capable of ejecting very small droplets.
2. Description of the Related Art
In ink-jet printers, it is desired that each ink droplet to be ejected from a printing head is as small as possible in order to improve print quality. From this viewpoint, an existing ink-jet printing head is capable of ejecting small ink droplets of about 2 pico-liter (pl) by, for example, modifying a control pulse waveform for an actuator to apply ejection energy to ink, or decreasing the diameter of each nozzle.
In recent years, however, it is required to eject very small ink droplets of less than 2 pl to realize higher-quality, higher-resolution print. By the above-described technique of modifying a control pulse waveform or decreasing the diameter of each nozzle, however, it is difficult to further decrease the size of each ink droplet.
Other than the above-described techniques, there is known a technique to regulate a control pulse waveform and, at the same time, to regulate a distance between the nozzle and a print medium such that a main dot (a main ink droplet) and a satellite dot (a satellite ink droplet), both of which are ejected through a nozzle in accordance with one pressure variation, may have substantially the same volume and such that landing positions of those two ink droplets may be different from each other (see Japanese Patent Application Laid-open No. 7-285222 (FIG. 1)). By this technique, the size of the main ink droplet can be decreased, besides the satellite ink droplet can be increased in size and thus this can be a dot independent of the main dot.
However, for printing an image at a very high resolution having, e.g., photographic quality, it is required to eject ink droplets each smaller than those obtained by the above-described technique. In addition to the requirement of ejecting very small ink droplets, there is a requirement for an ink-jet printer to eject very small droplets of conductive paste so that a very fine electric circuit on a substrate can be printed.
U.S. Patent Application Publication No. US2004/0046825 A1, disclosed by the inventor, discloses an apparatus for ejecting very small droplets to form dots on a print medium, the apparatus characterized by including: a first droplet ejector capable of ejecting a first main droplet in a first trajectory and a satellite droplet smaller in volume than the first main droplet, the satellite droplet being ejected together with the first main droplet; a second droplet ejector capable of ejecting a droplet having a second trajectory intersecting the first trajectory; and a control unit for controlling the first and second droplet ejectors so that the first main droplet and the droplet ejected from the second droplet ejector collide to unite with each other and a united droplet flies in a trajectory different from the first trajectory and the satellite droplet lands on the print medium. In this apparatus, actuators are separately provided to drive the first and second droplet ejectors.
U.S. Pat. No. 6,167,748 (corresponding to Japanese Patent Application Laid-open No. 11-99651) discloses a liquid discharge method and apparatus which enables, for example, a gradation recording by ejecting two droplets respectively from first and second discharge ports and by colliding these droplets to unite with each other while these droplets are flying. The first and second discharge openings are provided with first and second flow paths which communicate therewith respectively, and first and second energy generating devices for discharging the droplets therefrom respectively. On the other hand, Japanese Patent Application Laid-open No. 2001-239681 discloses that a plurality of droplets discharged from a plurality of nozzles are made to unite with each other to land on a recording paper. Each of the plurality of nozzles communicates with a flow path provided with a heater.